Among green energy technologies, solar energy is a maturer one. Solar cells are usually used to directly convert light energy into electric energy. For example, the solar cells on an electronic calculator and the solar cells on the roof of a house directly convert light energy into electric energy, and the electric energy is stored for the following usage. From 1960s, satellites have adopted solar cells as the energy source. From 1970s energy crisis, solar energy began its civil application.
Solar cells may be based on three effects: (1) Photoelectric effect: Photons incident on the surface of a metal hit electrons of the metal, and the electrons absorbing sufficient photons energy then leave the metal to be free electrons in vacuum, and the free electrons become photocurrent under an external voltage; (2) Dember effect (Photodiffusion effect): Photons incident on a semiconductor excite electron-hole pairs, the electron-hole pairs are converted into photocurrent; (3) Photovoltaic effect: Photons incident on a p-n diode induce a potential difference at two sides of the p-n junction. At present, solar cells are usually based on the photovoltaic effect.
The current focus of solar energy researches is to increase the efficiency thereof. In illumination and display applications, solar cells and light emitting elements are separately installed. For example, the traffic signs, advertisement signboards or indicators need solar cells converting light into electric energy, secondary batteries storing electric energy, a light emitting component, a circuit board controlling the abovementioned components. However, the many components increase cost, waste space, increase environmental burden, which all conflict with the aims of green energy.